Carrot (Daucus carota) - Alternaria Leaf Blight
Cause The fungus, Alternaria radicina (syn. = Stemphylium radicinum), can be seedborne or may overwinter in crop refuse. A. radicina typically is responsible for most cases of black root rot under cool soil or storage conditions.
Symptoms In Oregon, black root rot typically occurs either in late winter or in early spring on overwintered carrots or carrots grown for seed production. The fungal pathogen attacks the leaf-stalk base and crown and may infect in the fall to incite late-winter and spring lesions. Symptoms may be small to extensive black lesions on the root surface, with a well-defined border between infected and healthy tissues. Such lesions may not penetrate deeply. Lesions may enlarge slowly in colder temperatures, even those near freezing. If lesions are extensive, plants may be stunted and also are prone to secondary infections by other organisms. In addition, black root rot may occur on carrots in storage, but the decay does not appear to spread from root to root in storage.
Cultural control Together, these measures should keep the diseases under control.
- Use pathogen-free seed.
- Irrigate so that carrots are dry by nightfall.
- Rotation out of carrots for at least 1 year is important because refuse is the main way the organisms overwinter. Plow under crop debris soon after harvest.
- For seed crops, obtain transplant carrots free of Alternaria infection.
Chemical control Good eradication of A. radicina from carrot seed has been achieved with hot water (122°F for 30 min).
- Nevado 4F at 1 pint/100 lb seed plus a dye. For Washington and Idaho only. 24-hr reentry.
If the disease is common, spraying may be necessary. Rovral applied in fall may prevent black rot on roots of seed carrots, although few data are available to verify this.
- Copper products. (Group M1)
- Bordeaux 6-6-100.
- C-O-C-S WDG at 1 to 1.9 lb/A. 48-hr reentry.
- Dithane F45 Rainshield at 1.5 quarts/A on 7- to 10-day intervals can be used on seed crops only. Washington and Oregon only (SLN WA-090020, SLN OR-090016). 24-hr reentry.
- Rovral 4 Flowable at 1 to 2 pints/A in a minimum of 10 gal/A water on 7- to 14-day intervals. Do not make more than 10 applications a season if tank-mixed with another fungicide labeled for Alternaria control, only four (4) applications per season if used alone. Can be applied up to the day of harvest. 24-hr reentry.
References Farrar, J.J., Pryor, B.M., and Davis, R.M. 2004. Alternaria diseases of carrot. Plant Disease 88:776-784.
Pryor, B.M., Davis, R.M., and Gilbertson, R.L. 1994. Detection and eradication of Alternaria radacina on carrot seed. Plant Disease 78:452-456.